The trainer in my gym told me that from doing squats you'll get a bigger butt. But that's something that I don't want, because of my body type I already have a big butt. :) He advised doing leg press exercises instead. Other things that would speak for doing LP exercises were (as he told me) that you can shift your feet (not during the exercise of course). By placing them wider you'll be training the abductors more, it would be better for the hipbone, because the LP would not put so much pressure on the hipbone as the squat. If you've trained your back the day before you can let the back rest. I am acquainted with both exercises how to do them correctly. What would you suggest, so that my glutes won't be getting bigger?
Like any muscle, the glutes can get bigger from weight training when adequate calories are consumed. Any basic exercise that includes the glutes can increase muscle mass, including leg presses, particularly if particular dietary restrictions are not implemented. The effects you get from your program can largely be affected by diet as well as exercise. It is, however, possible to seemingly reshape your glutes by simultaneously decreasing body fat and restoring muscle mass; tightening this area and creating the illusion of a higher, more shapely rear end.
If you are not prepared to implement certain dietary changes to lower your body fat or if you are already quite lean, leg presses with the feet wider and not high on the platform would certainly be one strategy to de-emphasize the development of the glutes. Keep in mind though, at some point you may want to try other exercises other than leg press, if not for continued progress, for variety's sake, so workouts do not become so mind numbingly boring.
If you are on a split program, consider performing one compound exercise for the glutes and quads (leg presses for example) plus an additional isolated exercise for the quads (such as leg extensions).
If you choose to perform squats, it is also possible to slightly vary your biomechanics, so more emphasis is placed on your quads and less on your glutes. High bar squats will place relatively less emphasis on the glutes and more on the quads than low bar squats. Examine your form for the relative torque at the knee versus the hip. See Squat Analysis. Also, make sure you are not bent over at the hip at the top of the squat. Consider extending your hips slightly sooner than your knees if this is the case. Also, realize a deep squat or leg press with fuller range of motion will emphasize the glutes. For this reason, consider half squats. As suggested, a wider stance may also decrease the relative evolvement of the glutes and quads by decreasing the torque forces on hip and knee extension, a modification you can do with many compound movements such a squats or leg presses. The wider stance will indeed place more force on the adductors, particularly the ischial fibers of the adductor magnus on both movements.
Exercises such as the front squat emphasize the quads and de-emphasize the glutes by decreasing the torque of the hip relative to the knee. Your knees may be at risk if you alter your form too much though (eg: allowing your heels to come off the floor or platform, placing your feet too low on leg press, etc), particularly if they are not given the opportunity to adapt adequately to the greater torque forces. See Adaptation Criteria. Keep in mind, your exercise intensity may be compromised if you shift too much emphasis to the quads and away from your glutes. Exercise intensity appears to be a key factor in increasing the metabolism hours after anaerobic exercise.
An alternative solution is to perform higher reps (12-15 reps) on your glute / quad exercise and in addition, and perhaps in addition, perform HIIT two to three days per week. This will allow you to achieve the fat burning benefits of performing anaerobic exercise with the largest muscles of the body while de-emphasizing the muscle building effects of a moderate repetition range.
Obviously, if you tend to carry some of your body fat in the rear quarters, a reduction of body fat may reduce its proportion somewhat, although the hips and glutes may appear to be one of the last places fat seems to be reduced in women (see Spot Reduction Myth). Take regular circumference measurements and body composition tests so you will be able to make immediate modifications to your program, particularly your diet, dependent upon the changes of fat, lean body weight, and girths. See dietary development.
In summary, examine your exercise biomechanics and possibly slightly alter your form. Only perform a single exercise involving the glutes and change exercises periodically. If your posture or muscular proportions can be improved, regularly perform some of the corrective exercises for lordosis or an additional exercise for the hamstrings, respectively as needed. Finally, monitor your body composition and girths, modifying your program accordingly.
Also see Reducing Chuncky Thick Thighs.